The invention resides in a method for operating a servo press and in a press equipped with a servo drive.
For the transformation of massive metal components as well as for the deformation of metal sheets increasingly presses are used whose plungers are driven preferably by an electric servo drive. In this respect, DE 10 2006 033 562 B3 discloses presses whose plungers are driven by servomotors. The servomotors provide for the respective plungers an up and down stroke movement in accordance with a predetermined guide angle/position-curve. With the programmable control of the servomotors guide angle/position-curves can be achieved which to a large extent or even precisely approximate curves as they were generated in the past by mechanical presses. Also novel curves which can not be realized with mechanical presses can be achieved. The servo drive follows the predetermined curves as precisely as possible. To this end the servo drive is generally controlled on a position basis.
Such presses are also known from DE 10 2004 009 256 B4. This printed publication discloses the drive of a press plunger via an interposed eccentric drive by several servomotors. Of the servomotors at least one is operated on a position control basis. In this way, the desired guide angle/position-curve is obtained which is also designated travel distance-time curve. The printed publication also discloses to operate one of the motors on a position control basis and the other motor on a force or torque control basis. In this way it is avoided that individual servomotors operate in opposition to one another because of contradicting position measurements. It is made possible thereby that, overall, the predetermined travel distance-time-curve is maintained.
Between the plunger and a press table, that is driving the transformation of a work piece, generally substantial forces are effective which result in an elastic deformation of the press when the plunger reaches the lower dead center position. The press table, the plunger and the press head part may be subjected to elastic bending. The press frame and the crank may be elastically stretched or compressed. With this elastic deformation a substantial deformation energy is stored in the components mentioned which energy is again released to drive the plunger after it has passed the lower plunger dead center position. The press drive then acts as a brake and moves the plunger away from the press table in a controlled manner while dissipating the elastically stored energy.
In a servo press, this braking energy must be accommodated by the servo drive. This is an additional load for the servo drives. Additional forces to be accommodated by the servo drive may originate from the tool or the manufacturing process. For example, nitrogen springs (uncontrolled) drawing equipment, ejectors or similar devices may be provided, for example arranged in the plunger. Such equipment is effective over a relatively long distance and result in the above-mentioned increased load.
It is the object of the present invention to improve the operation of a servo press with a press plunger and a servo drive for driving the press plunger.